1. Technical Field
The present invention relates to an optical receiver and, particularly, to an optical receiver using an avalanche photodiode.
2. Related Prior Art
For preventing an avalanche photodiode (APD) from being broken by overcurrent, there are known optical receivers configured to detect the optical power of input light received by the APD and to control the bias voltage applied to the APD.
The Japanese patent application laid open as JP-H11-186971A or the Japanese patent application laid open as JP-2004-289206A discloses a circuit for letting a current mirror circuit produce a mirrored current which reflects a photocurrent flowing in the APD, and for converting this mirrored current into a voltage. An analog-to-digital converter (A/D-C) converts this voltage into a digital signal and the bias voltage supplied to the APD is controlled based thereon.
Normally, the optical receiver is connected to a host system and is configured to output information about the optical power of the input light received by the APD, to the host system. For this purpose, the optical receiver calculates the optical input power from the aforementioned digital signal. In order to enhance the detection accuracy at low optical power, it is preferable to increase the resistance of a sensing resistor.
However, an input voltage range that can be converted into a digital signal by A/D-C is dependent on a reference voltage supplied to the A/D-C, and the increase in the resistance of the current sensing resistor will result in increase of the input into the A/D-C, which causes saturation of the output of the A/D-C. Conversely, if the resistance of the current sensing resistor is decreased in order to widen the detection range of the optical input power, the optical detection accuracy will be lowered at low optical powers.
Therefore, the present invention provides an optical receiver that accurately detects low optical powers and has a wide dynamic range of optical detection.